Chemical agent monitors-detectors are known to be used for detection of chemical warfare agents for battlefield, for environmental clean-up, for treaty monitoring and verification missions, and for other purposes, where chemicals contained in the air are to be detected.
The chemical Agent Monitors ("CAMs") are employed by several armies as a reliable system for the above-described purposes.
The Improved Chemical Agent Monitor ("ICAM"), as best shown in FIG. 1, maintains the performance characteristics of the CAM but has an improved design which is even more reliable and maintainable.
A recently-developed Improved Chemical Agent Monitor-Detector ("ICAM-D") incorporates the ICAM to provide stand-alone detection capability using the proven, type-classified ICAM as the core of the detector. The ICAM becomes the sensor for the ICAM-D while the detector module provides command and control functions.
CAM, ICAM, ICAM-D and their modifications employ a dual polarity Ion Mobility Spectrometer ("IMS") as a sensor. In an IMS-based apparatus for an air sample analysis (CAM, ICAM, ICAM-D and the like) incoming gas from an inlet of the apparatus permeates a membrane and enters a drift tube. The drift tube houses a cell, which consists of an ionizing chamber (or reactor region), an electronic gate (or a shutter grid), and a drift region that terminates in an ion collector (or a charge collector).
Being introduced into the reactor region, the various molecules contained in the air sample are ionized by an ionizing source, and the ions of each polarity are separated by the electric field applied to the reactor region. It will be appreciated by those skilled in the art that certain electric fields are applied to all systems of the cell from a power source. An electric field applied to the shutter grid opens an entrance to the drift region only to ions of the polarity to be analyzed. Each species of the polarity being analyzed has a distinguishing ion mobility which determines a time of "flight" of the ions through the drift region to the charge collector. As each ion (or ion cluster) strikes the charge collector, it is represented as a peak in a collected current waveform. The collected charge produces a spectrum of the peaks in time. Each peak carries information regarding the identity and concentration of the chemical(s) being detected. Each peak is assessed by a microprocessor system programmed with a detection algorithm that recognizes the chemical and determines its concentration.
Despite the advantages of the CAM, ICAM, ICAM-D and their modifications developed recently, they all have a serious problem still unavoidable in dual polarity IMS-based detectors. This problem is associated with switching a polarity in the cell in order to analyze the chemicals of the opposite polarity. After the polarity in the system is switched, the previous ("old") background ions are replaced by the succeeding ("new") opposite polarity background ions. During the time that this replacement takes place, the "new" background ions lose their charge to the remaining background "old" background ions and are not available to contribute to the results of the analysis, thereby upsetting these results. Therefore, it would be highly desirable to overcome this disadvantage of the prior art, while enjoying high performance characteristics of the CAM, ICAM, ICAM-D and their modifications.